When I think of a minilesson, I think of something that is pertinent for students to have a working knowledge of, but not something that would require a full unit to discuss. Whenever I had minilessons in school, it was always to go over a common grammar error that we had made in our papers. For example, if several students had problems with tense agreement in their papers, then we would have a minilesson one class on how to make sure that your tense remains the same for a sentence, paragraph, etc.
3 possibilities for a minilesson:
tense agreement
proper use of prepositions
the whole shebang on whether you underline, quote, or italicize a title